1. FIELD OF THE INVENTION
This invention relates in general to a process for the treatment of parylene films and coatings. In one aspect, this invention is directed to inducing fluorescence on the surface or outer layer of parylene films and coatings, such as parylene coated electronic components, by exposure thereof to an active plasma and the utilization of such fluorescence for quality control inspection purposes. In a further aspect, this invention is directed to a process for the treatment of parylene wherein selected sites are rendered fluorescent when exposed to an active plasma utilizing certain electrically excited gases. In a still further aspect, this invention relates to articles coated in whole or in part with parylene, such as negotiable instruments, identification cards, films, disks and the like, which are caused to have certain sites of a predetermined configuration capable of fluorescing when exposed to light of the proper wavelength. The invention is also directed to a process for the removal of plasma induced fluorescence.
2. BACKGROUND OF THE RELATED ART
Parylene is a generic term applied to the class of polymers, the poly-p-xylylenes (I), which are prepared most conveniently from a dimer, a [2.2]paracyclophane (II), by the process of vapor deposition polymerization. ##STR1## wherein R is hydrogen or chloro, with at least one R being chloro, and n has a value of from 1 to 4.
When deposited on a substrate, parylene forms a continuous, inert, transparent, conformal coating which has excellent physical, electrical and barrier properties, excellent resistance to chemical attack, and which retains these attractive properties to relatively high temperatures. Due to its ability to provide films of uniform thinness and conform to substrates of varied geometric shapes, it is ideally suited for use as a conformal coating in a wide variety of fields, particularly in the electronics industry.
The term "parylene" as employed throughout the specification and appended claims is intended to encompass not only the halogenated poly-p-xylylenes of formula (I) above, but other known parylenes containing one or more substituents on the aliphatic or aromatic portion of the recurring unit and which can be successfully rendered fluorescent by an active plasma.
The preparation of p-xylylene polymers by various routes has been reported in the patent literature. For example, in British patent number 650,947 which was granted Mar. 7, 1951, polymer formation was detected on the walls of a cooling chamber after p-xylene was vaporized and pyrolized. Those who later prepared poly-p-xylylene films by this general procedure described them as being inherently fluorescent.
Also in U.S. Pat. No. 2,719,131 which issued in Sep. 27, 1955 to E. I. DuPont de Nemours and Company there is disclosed a process for preparing poly-p-xylylene wherein the vapors of p-xylene gas were pyrolized in the presence of chlorine gas.
The preparation of para-xylylene polymers was also disclosed in U.S. Pat. No. 3,342,754 which issued on Sep. 19, 1967 to William Franklin Gorham and is assigned to Union Carbide Corporation. In this reference it is indicated that true linear homopolymers of para-xylylene could be produced in nearly quantitative yields by heating a cyclo-di-paraxylylene having up to six aromatic nuclear substituent groups to a temperature between 450.degree. C. and 700.degree. C. for a time sufficient to cleave substantially all of the dipara-xylylene into vaporous para-xylylene monomer and cooling the vaporous monomer to a temperature below its ceiling condensation temperature. Moreover, the higher purity poly-p-xylylene films prepared by the method of Gorham are substantially non fluorescent. The fluorescence reported in films prepared by earlier methods are presently thought to result from impurities introduced by the process.
As indicated above, prior to the present invention a wide variety of references have been available in the literature disclosing the preparation and use of poly-p-xylylene. One of the main utilities has been, and continues to be, its use as a conformal coating for a variety of articles, such as electronic circuit boards, and the like.
Parylene's use as a conformal coating for military circuit boards was recognized early, and its inclusion among the electrical insulation compounds qualifiable under the military specification governing coatings for printed circuit assemblies (MIL-I-46058) was a first order of business upon public announcement of the product in 1965. In the course of events, users of conventional coatings found that inspection of the coated board or assembly for holidays or pinholes would be greatly facilitated by the inclusion of a fluorescent dye in the otherwise clear and colorless coatings. The fluorescence from the dye would render the coated areas identifiable under an inspectors black light, but would otherwise would not be noticed. As the military requirements evolved, it became necessary for the coating manufacturer to provide the option of a fluorescent version of each coating at the users request. For conventional coatings this requirement was easily met by adding a small amount of any one of a number of commercial brighteners.
In the parylene vapor deposition polymerization process, however, a fluorescent additive compound such as a commercial brightener could conceptually either be added to the dimer at the outset of the process, or to the finished film. The parylene film barrier properties are such that diffusion of sufficient amounts of a fluorescent additive compound, necessarily a rather large molecule, into the finished film has not yet been successfully demonstrated. This process would be analogous to a dyeing of the parylene film, which also has not been successfully demonstrated. Practical considerations would further mitigate against such post deposition addition in cases where the substrate surface is convoluted, and especially when uniform distribution of fluorescence over all coated surfaces is required for the overall reliability of the inspection procedure it is intended to facilitate.
If the fluorescent additive compound is to be added to the dimer at the start of the process, it must be sufficiently volatile to pass through the process with the dimer and the product of its cleavage, the monomer, sufficiently condensible to deposit with the parylene during its deposition, and furthermore it must be sufficiently stable to survive the high temperatures of the process unchanged in its ability to create fluorescence. These conflicting requirements severely limit the field of choice among known fluorescent additives.
The first compound shown to be useful as an additive to dimer to produce fluorescence in parylene films and coatings was anthracene. Later, two members of the Calcofluor family of commercial brighteners, (Calcofluor is a trademark of American Cyanamid Co., Wayne, N. J.), known as Calcofluor White RW, RWP or SD, were shown to be still more effective as a dimer additive for producing fluorescence in parylene coatings. These compounds are 7-dialkylamino-4-methylcoumarins, and as such are chemically different from other Calcofluor family members.
Hence, prior to the present invention, fluorescence in parylene by pre-deposition intervention was achieved specifically by the addition of a very limited number of optical brighteners to the dimer at the time of the vapor deposition polymerization process. This, of course, results in the entire parylene coating exhibiting fluorescence when methods for detecting its brightness are employed. By these processes it was not possible to create or impart fluorescence only at predetermined sites in the parylene coating.
The fluorescence produced by predeposition intervention, i.e., the addition of a fluorescent additive to the dimer at the outset of the process, while not patterned in the sense of a predetermined pattern, actually varies in intensity from place to place in the deposition chamber. The fluorescent additive is not transported through the deposition chamber as uniformly as the monomer, and tends to be deposited more heavily on substrates which were located on the line of sight from the generator nozzle. Thus, while the intensity of the fluorescence in the treated parylene article varies from place to place, for practical purposes the entire surface exhibits at least some degree of fluorescence.
Although studies had been made of the possibility of producing fluorescence by post deposition methods, no success has yet been realized in the permanent incorporation of a fluorescence additive in a parylene film by diffusion, or by a dyeing procedure. Such a procedure, if effective, would be a means of rendering selective sites of parylene coatings fluorescent. While such selective creation of fluorescent parylene coating sites may be of minimal interest in the inspection of coating quality on printed circuit assemblies, other uses abound. However, prior to the present invention, the ability to create specific sites of fluorescence in parylene coatings using an active plasma was unknown.
It should also be noted that at least one of the compounds which could be utilized to impart fluorescence to parylene films and coatings, namely anthracene, is a suspected carcinogen and hence its use should be avoided.
Additionally, when the parylene film is very thin, it will usually contain insufficient dye to be clearly detectable. If larger proportions of the dyes are employed the quality of the film or coating may be adversely effected.
Another disadvantage using additives to impart fluorescence to articles comprised in whole or in part of parylene, is that the fluorescence can not be erased. In some instances, it might be desirable to erase fluorescence from certain areas of an article coated with parylene, for example, if the fluorescence has been used for coding or part identification and revision must be made.
Accordingly, one or more of the following objects will be achieved by the practice of this invention. It is an object of this invention to provide a novel method for the treatment of parylene. Another object of this invention is to induce fluorescence in certain films and coatings by exposure to an active plasma utilizing certain gases. A further object is to provide a quality control method for inspecting certain parylene films and coatings utilizing plasma induced fluorescence. Another object of this invention is to provide a process for treating parylene wherein selective sites of fluorescence are imparted to the parylene. A further object is to provide a novel process for selectively imparting fluorescent sites to articles comprised in whole or in part of parylene. A still further object of the present invention is to provide a method for the treatment of Parylene C. Another object of this invention is to provide a process for the treatment of Parylene D. A further object is to provide articles comprised of parylene which exhibit selective sites capable of fluorescing upon exposure to certain light excitation means. Another object is to create fluorescent sites of a predetermined configuration in parylene coatings. Another object of the present invention is to use plasma induced fluorescence to detect where parylene has leaked onto unwanted areas. A still further object of the present invention is to provide articles such as negotiable instruments, currency, and the like, which have a parylene coating containing sites of a predetermined configuration which fluoresce and hence provide for identification or authentication. Another object is to induce fluorescence in parylene films and coatings which, if desired, can easily be erased using an oxygen plasma. These and other objects will become apparent to those skilled in the art in the light of the teachings herein set forth.